•We carried out vegetation studies in the ancient oak forest Hasbruch in Germany.•We analysed drivers of changes in vascular plant species richness.•We analysed the effect of management and vegetation type on these changes.•The main drivers for changes in species richness were light and water availability.•Effects of the drivers strongly depended on the vegetation type. We carried out vegetation studies in the ancient Hasbruch forest, which provided the unique conditions of unmanaged (UM) and managed (M) stands in two vegetation types Stellario-Carpinetum loniceretosum (POOR) and stachyetosum (RICH) stands in one closed forest allowing us to study (1) the changes in species composition and richness over 20 years considering the entire forest as well as (2) group-specific changes. Hasbruch forest in the lowlands of Lower Saxony, Germany. In 2016 we resurveyed the vascular plant species composition of 79 semi-permanent plots analysed in 1996. General and group-specific trends as well as drivers of changes were analysed using DCA, PCA, LM, t-test, U test, ANOVA. Tree and shrub layer coverage increased significantly in the entire forest. Herb layer species richness decreased significantly only in group RICH_UM. While the pH-value increased significantly in group POOR, it decreased significantly in group B. EIV F increased significantly in group POOR_M and decreased significantly in group RICH_UM. EIV L increased significantly in both of the managed groups, while it decreased significantly in group RICH_UM. An increase was found for EIV N in group POOR_M, whereas a decrease was found in group RICH_M. Strongly increasing species were Ilex aquifolium, Rubus fruticosus agg. and Hedera helix; strongly decreasing species Geum urbanum and Primula elatior. The change in species richness was positively affected by ΔpH and negatively affected by ΔT1cov in the entire forest. The main drivers for changes in species richness and composition in the Hasbruch forest were light and water availability. The effects of the drivers strongly depended on the vegetation type. Changes in species composition were more pronounced in nutrient-rich forests than on nutrient-poor sites. In nutrient-rich forests, decreased groundwater influence led to decreased soil pH which especially affected typical woodland plants in a negative way. Management positively affected light-demanding species as well as some N-demanding species. Thus, unmanaged, nutrient-rich stands displayed the highest losses in species diversity. In the nutrient-poor stands, changes in species composition were not significantly related to changes in soil and management. However, increasing pH as well as increased Ellenberg L, F, and N values suggest a tendency towards eutrophication. This is possibly a consequence of N deposition and recovery from soil acidification.